A display panel, such as a liquid crystal display (LCD), includes pixels arranged in rows and columns. Each pixel has its own electrode for receiving a driving voltage. The voltage level of the driving voltage corresponds to the brightness of the pixel. More particularly, the amount of light passing through or reflected by each pixel is controlled by the level of these driving voltages.
A display panel further includes a driving circuit to receive pixel data corresponding to the brightness of the pixels of the display panel, generate driving voltages according to pixel data, and provide driving voltages to each of the pixels. When a display panel displays video images, the pixel data of different video images may be different. In this condition, the brightness of the pixels is controlled by the driving circuit by changing levels of the driving voltage applied to the pixels. There is a transition time period for the driving circuit to change levels of the driving voltage. The transition time period is between the time periods to display two successive video images, e.g., a previous video image and a current video image. In many applications, the driving circuit is required to change levels of the driving voltage in a very short time. In this condition, the transition time period can be much shorter than the time period to display each of the video images. The shorter the transition time period, the faster the transition rate of the display panel is and the better the display quality of the display panel will be.
However, for some types of display panels, such as electrophoretic displays (EPD), the transition time period required by their driving circuits to change levels of the driving voltage can be longer than that required by driving circuits used in liquid crystal displays. In addition to longer transition time period, the driving circuit used by an electrophoretic display must control the length of the transition time period and the levels of the driving voltages more accurately than the driving circuit used by a liquid crystal display during the transition period. The display quality of a conventional driving circuit used by an electrophoretic display will be degraded if the transitional conditions are not well-controlled. Therefore, conventional driving circuits used with liquid crystal displays are not suitable for use with electrophoretic displays since they cannot control the length of the transition time period and the levels of the driving voltages during the transition time period as accurately as required by electrophoretic displays. Examples of conventional driving circuits are described below.
One example of a conventional driving circuit is shown in FIG. 1 of U.S. Pat. No. 6,097,219 entitled “OUTPUT BUFFER WITH ADJUSTABLE DRIVING CAPABILITY” of Urata et al., which is reproduced as FIG. 1 herein. With reference to FIG. 1, the length of a transition time period required by a driving circuit 100 to change levels of driving signals is determined by the number of buffer circuits, i.e., B0, B1, B2, and B3 which are turned ON in response to each of bit signals b1˜b10. The greater the number of buffer circuits turned ON in response to each of bit signals b1˜b10, the shorter the transition time period that is required by driving circuit 100. Driving circuit 100 does not control the levels of the driving voltages during the transition time period.
Another example of a conventional driving circuit is shown in FIG. 1 of U.S. Pat. No. 4,797,579 entitled “CMOS VLSI DRIVER WITH CONTROLLED RISE AND FALL TIME” of Lewis, which is reproduced as FIG. 2 herein. With reference to FIG. 2, the length of a transition time period required by a driving circuit 200 is determined by VP(t) and VN(t) provided by voltage generators 210 and 220, respectively, and the levels of driving voltages during the transition time period are determined by capacitance of an output capacitor Co. However, the length of the transition time period and the levels of the driving voltages during the transition time period are limited by the characteristics of a PMOS QPO, an NMOS QNo, and an output capacitor (Co) of driving circuit 200.
In addition to a voltage-driven driving circuit such as the conventional driving circuits shown in FIGS. 1 and 2, in some specific circumstances, a current-driven driving circuit, instead of a voltage-driven driving circuit, may be more suitable to drive a display panel. One example of a conventional current-driven driving circuit is shown in FIG. 1 of U.S. Pat. No. 6,417,708 entitled “RESISTIVE-LOADED CURRENT-MODE OUTPUT BUFFER WITH SLEW RATE CONTROL” of Fiedler, which is reproduced as FIG. 3 herein. With reference to FIG. 3, the length of a transition time period required by a driving circuit 300 is determined by the currents provided by two adjustable controlled current sources 310 and 320. Since driving circuit 300 is a current-driven driving circuit, it does not control the levels of driving voltages during the transition time period.
For conventional driving circuits shown in FIGS. 1, 2, and 3, although they appear to control the transition time period, the levels of driving voltages, or the levels of driving currents during transition, their control capabilities are sensitive to various factors such as voltage levels of driving voltages, current levels of driving currents, operating temperature, manufacturing variations, etc. As a result, such conventional driving circuits may not accurately control both the length of the transition time period and the voltage levels of the driving voltage when changing driving voltages from one voltage level to another during transition.
There is thus a general need in the art for a circuit with improved transition control for driving a display panel that overcomes one or more of the deficiencies of conventional driving circuits.